This environment has been dropped from Intel releases of Mac OS X, but thanks to SheepShaver, you can still set it up yourself on Mac OS X, Linux, Windows, and even BeOS if you want to.
Remember MacOS 9, or Classic as Apple named it once Mac OS X was released? On PowerPC Macintosh machines, you can install a Classic environment which launches a virtualised instance of MacOS 9 whenever you launch a Classic application.